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Vacuum Conveying Provides Material a la Carte

In the food industry an average of 70% of ingredients are processed as solids, typically as powders or granules. The range varies from super-fine icing sugar to coarse bacon pieces. These bulk materials show, particularly during transport, a wide range of behavior characteristics due to their particle sizes and shapes, water and grease content, and varying bulk densities. Therefore, the design of a practical conveying system requires an experienced supplier who has, over several decades, built up a deep knowledge in this field. If proven manufacturing know-how is combined with the ability to test the specific powder or granule, even the most difficult bulk materials can be transferred. To illustrate this, the following case study describes how manufacturers can gain many advantages in production by using the latest techniques.

A major food manufacturer, with more than 30 years producing freeze pops (flavored-water ice) and popcorn, provided an interesting case. Its range of customers is divided between leading national and international retailers, and discount and food wholesale companies. Depending on the season, up to 200 employees produce around 10 tons of popcorn each day.

The classic popcorn is made using the wet-roasting process by adding sugar and, depending on the requirements of the customer, other flavoring. This leads to the need for a high degree of manufacturing flexibility in order to fulfill customers’ needs, together with a corresponding demand for a conveying method that can easily adapt to the flavor of the day—or hour—as the case may be. Bulk material transfer is needed at several different locations, thereby requiring the conveyor to readily change between the various transfer tasks and capacities. Moreover, the conveying system must fulfill the demands of the International Food Standard, since the company is a certified food producer.

For production manager Juergen Regener, the question arose as to how the existing material flow could be improved for the different types and varieties of sweet corn to be roasted, as well as the crystal sugar, which are both necessary for popcorn production.

Bulk Bag Unloading

The largest volume of daily solids handling is the various grades of sweet corn kernels that are delivered in pallet-loaded bulk bags. The selection of sweet-corn grades, as well as the appropriate sugar, must then be transferred to the roasting machines in the remote production area. As is often the case in many older food-production areas, the existing material transfer methodology was very basic. It used the sequential manual delivery of individual bulk bags from the storage area, via an elevator, to the point of use in the first-floor production area. This method of handling not only requires a great deal of time and effort, but it is also problematic with respect to hygiene considerations, thus making the established process unacceptable. Moreover, following the elevator transport, the bulk bags were emptied manually by scooping with 10 L buckets. This unusual, labor-intensive practice was the only possible way to unload the bulk bags in this location due to the space limitations. In the immediate production area it was impossible to install complex bulk-bag unloading stations and lifting devices. The manual emptying required extra staff and, bearing in mind the constant increase in production tonnage, demanded heavy and repeated lifting from the operators.

The final consideration was the large quantity of processed sweet corn that results in a considerable accumulation of waste packaging material, which is naturally undesirable in the production area. Similar inconveniences existed with the handling of the crystal sugar that was delivered and processed from 50-lb paper sacks. The manufacturer sought to eliminate the various intermediate transport steps and investigated methods of automatically transferring the sweet corn and sugar directly from storage to point of use in the production area.

Evaluating the Conveying Methods

At first glance, it appeared that a number of mechanical conveying methods could be used to solve this transportation problem. Screw conveyors, in principle, are able to move an easily conveyable bulk material over a short distance, but can cause many problems in the food industry. Like most facilities, this manufacturer has a twice weekly, entire production area wet cleaning, which encompasses all machinery. Using the screw conveyors, together with all the mechanical parts involved, would mean an unreasonable effort in terms of mounting, cleaning, and reassembling. Additionally, the sweet corn requires gentle transportation, which is a disadvantage for the mechanical rotating screw since there is the risk of potential product damage.

In order to improve production flexibility, the client requested the option of unloading the bulk bags from the top (filling spout) as well as from the traditional bag-unloading station. In a similar way, the fixed layout and structure of the production building had to be considered, in this case over 65 ft total distance and 25 ft of conveying height. Since there was no accessible straight line between the material pickup point and the discharge and loading point, several intermediate direction changes were needed to accommodate the building, and thus made the conveying task even more difficult.

After concluding that screw conveyors were not suitable, the next possibility appeared to be a bucket elevator. This conveying principle is, in general, gentler than mechanical screw conveyors. However, the question remained as to how to load the buckets from the bulk bags. The aim was to avoid unnecessary handling. Moreover, the excessive requirements in terms of cleaning were still a problem. The installation envelope dimensions of a mechanical bucket conveyor would also require significant changes to the building structure, including the need for the roof to be modified.

Taking all factors into account, the customer’s specification for a conveying solution described a system that must be flexible, easily cleaned, able to empty bulk bags from either the top or the bottom (via a hopper), and able to convey various grades of sweet corn in a gentle fashion. All this needed to be done over a total conveying distance of 65 ft with an elevation change of 25 ft, along with several 90° direction changes. The system is required to handle a capacity of 1100–1750lb of sweet corn per hour. Simultaneously, the shortcomings of conventional mechanical conveyors were considered in order to avoid a capital equipment investment that simply shifted problems to another area.

Transport by Vacuum Conveyor

Unloading bulk bags from above

Following some research on the Internet and in industrial journals, the customer identified Volkmann, a German manufacturer whose core activities in the field of pneumatic vacuum conveying of bulk materials have spanned decades. With its own development, production, and thorough demonstration and testing facilities, together with daughter companies in Great Britain, France, and the United States, and a worldwide distributor network, the company has refined the principle of vacuum conveying to ideally fit the requirements of the client for sweet corn and sugar conveying. Depending on the demand, it is possible for a single vacuum conveyor to transfer three different types of sweet corn directly from the delivered bulk bags in the storing area.

For unloading the bulk bags from the top, special suction wands were developed to ensure that the bulk bag itself is not sucked onto the wand, thereby preventing transfer. After the initial pickup under the vacuum, the transfer takes place in a flexible suction hose, which requires little space by itself. This allows easy routing of the conveying line around the given structure of the building and through two stories into the production area. This simple 65-ft-long hose has no internal built-in parts and can be washed and rinsed easily. The ultimate vacuum conveyor, complete with integrated vacuum pump, is located at the end of the vacuum line.

This vessel is designed and manufactured entirely from stainless-steel modules, which allow for an optimized, individual setup for the respective conveying tasks, as well as easy dismantling and no-tools-required cleaning. Crucial for a safe, reliable, and maintenance-free operation is the selection of the vacuum pump. A multiple-stage, compressed air–driven Multijector vacuum pump creates the necessary vacuum flow rate and essential vacuum level directly at the receiver. That’s the point where the pump is only required to run intermittently, not continuously, as is the case with vane, positive-displacement, and other vacuum sources. The elimination of vacuum losses given by long vacuum lines, and the deliberate avoidance of alternative vacuum sources that have to be operated continuously, leads to low total energy consumption. With the Multijector, the entire vacuum conveyor is compact and self-contained, thereby allowing for use at various locations, even in production areas where space is restricted. In this case, the same vacuum conveyor can transport the different grades of sweet corn as well as the crystal sugar.

Shared Experiences

Although the advantages of the vacuum conveying technique are clear, the client wanted Volkmann’s engineering advice for the detail configuration of the system. Experience has shown that even bulk materials with the same name or chemical properties can demonstrate surprisingly different behavior during transportation. Close to 3000 documented 1:1 scale design trials have been carried out to date in Volkmann’s test centers, both in Europe and the United States. This wealth of experience is used as a design aid for new applications, and particularly for difficult and non–day-to-day bulk materials. Examples such as cream-fat-powder, with up to 70% fat content, cherries in alcohol, or a mushroom-water mixture, speak for themselves. Important basic parameters such as bulk density and particle size, as well as explosionproof zone rating checklists are available and should be used to gain a quick overview of the conveying task.

Multiple vacuum receivers at the end of the 65-ft conveying run

The client defined its parameters in the checklist and accordingly a 1:1 scale trial was set up in the test center. The principal conveyability was not questioned with the given materials. However, it is always recommended to perform the conveying under original conditions (here, 65-ft total distance, 25-ft height) in order to gain the required capacity with an economic safety factor, as well as to ensure the lowest energy consumption.

Operation managers like Regener appreciate witnessing the conveying system of their choice in full, true operation, and to acknowledge the overall performance before changes are made in the production process. With difficult bulk materials, long-term performance is demonstrated by circle conveying. In Volkmann’s test center, conveying distances of up to 400 ft and conveying heights of up to 165 ft have been achieved. On shorter distances, and on bulk densities and particle sizes, capacities of up to 12 tn/hr are possible.

During the joint design and test phase of the case study, investigation of the raw sweet corn material identified that certain amounts of fines (dust) are present. Such fines are undesirable during the subsequent roasting process. The vacuum conveyor is equipped with an additional feature to prevent such fines. Following the transfer along the 65-ft conveying distance, and at the end of the line, the fines are separated from the coarse sweet corn and are collected in a second dust receiver.

During a further expansion of the production lines, the vacuum conveyor was equipped with an additional conveying line coming from the storage area of the crystal sugar. Sweet corn and sugar are now conveyed through separate lines. This flexibility shows a further advantage against rigid, mechanical transportation methods—adaptability to future demands.

After more than 24 months of operation with the new vacuum conveying system, the positive impressions from the design and test phase have been completely verified, with Regener convinced of the reliable operation of the Volkmann vacuum conveyor. With the three distinctive suction ports in the sweet corn storage area, even mixtures of different sweet corn grades can be processed.

Finally, should a further increase in production capacity or a conversion of the buildings require a change to the layout again, Regener already knows that with a simple rearrangement of the vacuum conveying lines, he can utilize the advantages of vacuum conveying to automate material flow to the production lines.